1. Field of the Invention
The invention relates generally to improvements in phase-locked-loop systems, and more particularly, to a phase-locked-loop circuit configuration which eliminates the statistical nature of the acquisition process, thereby, affording a substantial reduction in, and predictability of loop lock-up-time.
2. Description of the Prior Art
A basic prior art phase-locked-loop is depicted in FIG. 1a. Initially, a signal coupled to input signal conductor 10 is applied to phase detector 12. This signal is compared in phase detector 12 with a feedback signal applied to conductor 14 by free-running controlled oscillator 16. Depending on the application employed, controlled oscillator 16 is disclosed in the prior art either as a voltage controlled oscillator (VCO), or a current controlled oscillator (CCO). Consequently, the output of phase detector 12 at conductor 18 is an error signal which is a function of the phase difference between the input signal at conductor 10 and the feedback signal at conductor 18. This error signal, in turn, is applied to loop filter 20, which conditions the error signal providing at conductor 22 a signal suitable for controlling controlled oscillator 16. The signal at conductor 22 is such as to cause a change in the frequency of operation of controlled oscillator 16 in a direction that reduces the error signal at conductor 18 to a minimum level. This is the classic locked condition of a phase-locked-loop and, accordingly, the output signal at conductor 14 tracks the input signal at conductor 10.
Acquisition or the time it takes a phase-locked-loop, such as the circuit of FIG. 1a, to lock up is statistical or random in nature. In some applications, such as standard radio communications and color television communications, the acquisition process is not of primary importance when weighed against overall system design goals. Thus, the basic phase-locked-loop of FIG. 1a would suffice, in the previously indicated applications, notwithstanding the statistical nature of acquisition. However, in some other communication applications, such as digital communication systems and space communication systems, it is important to be able to decode or demodulate the intelligence immediately, since a delay could mean the loss of valuable information; therefore, the acquisition process is of primary importance. In addition to fast acquisition, predictable acquisition is also important in systems of the type aforementioned. Thus, the acquisition process can be broken down into two components, i.e., speed of acquisition and the predictability of acquisition. The separate components, to some degree, are dependent on each other.
The statistical nature of the acquisition process can better be understood by referring to the phase plane diagram, for a first order phase-locked-loop, illustrated in FIG. 1b. Although the phase-locked-loop, previously described, in FIG. 1a, is a second order system, the phase plane diagram of FIG. 1b will suffice to explain the acquisition process. In FIG. 1b, .phi. is the phase of the feedback signal relative to the input signal, and .phi. is the rate of change of phase and is proportional to the error signal. Acquisition is defined, in the art, as the time required for a phase-locked-loop to reach a condition where the error signal is near zero at a stable null (depicted as .pi./2 in FIG. 1b). As is well known, some error signal is always necessary in order for the loop to track properly once acquisition is attained. Thus, as will be explained hereinbelow, theoretically, acquisition in the phase-locked-loop of FIG. 1a can vary from zero to infinity.
Concurrently referring to the basic phase-locked-loop of FIG. 1a and the phase plane diagram of FIG. 1b, a signal energizing the loop at conductor 10 has a probability of being in phase with free-running controlled oscillator 16. This in phase stable condition is depicted in FIG. 1b as the stable null .pi./2. There is also a porbability that an input signal, when initially received will be 180.degree. out of phase with free-running controlled oscillator 16. This condition is depicted in FIG. 1b as the unstable null at 3.pi./2 or the unstable null at -3.pi./2. In addition, there is a probability that the phase relationship between the input signal and controlled oscillator 16 is initially at some point between the previously mentioned limits and the stable null .pi./2. In actual practice, however, the loop will not stay at an unstable null for an indefinite time, primarily, due to system perturbations, such as noise. Accordingly, as is illustrated in FIG. 1b, controlled oscillator 16, at some point in time, will be driven off of the unstable nulls following the path indicated by the arrows. For example, if controlled oscillator 16, after the application of an input signal, is instantaneously at either of the unstable nulls 3.pi./2 or -3.pi./2, controlled oscillator 16 will stay at one of the previously mentioned unstable nulls. But given a change in system conditions, such as frequency or noise, controlled oscillator 16 will be driven off of the unstable null, following the path indicated by the arrows, increasing its velocity .phi., and finally, arriving at the stable .pi./2. However, the time it takes to accomplish the foregoing is indeterminate, since there is no assurance of how long the system will stay locked on an unstable null. Of course, once the system is driven from an unstable null, the time it takes to reach a stable null depends on the inertia in the sytem and can be determined to a degree of certainty by the parameters of the system. Computer studies have been made, given a random input signal, to determine the probability of acquisition for phase-locked-loops of the type depicted in FIG. 1a. For example, see, S. L. Goldman's article, entitled "Second-Order Phase-Locked-Loop Acquisition Time in the Presence or Narrow-Band Gaussian Noise," IEEE Transactions on Communications, 297-300, October, 1973.
There are methods, according to the prior art, to improve acquisition. One well known method is to increase the loop bandwidth. As is known to those with skill in the art, increasing the bandwidth of the loop increases .phi., the rate of change of phase. As previously mentioned, .phi. is proportional to the error signal; therefore, increasing the bandwidth of the loop improves acquisition since an increase in .phi. increases the loop acceleration allowing the loop oscillator to approach a stable null more quickly (see FIG. 1b). Nevertheless, there is still a probability during initial acquisition to be on an unstable null. Accordingly, there is still an uncertainty, even though the uncertainty has been improved, as to how long the system will remain on an unstable null.
Another method, that is well documented in the prior art, is the utilization of a sweep frequency. This method definitely improves acquisition, especially if the phase-locked-loop is on an unstable null initially. On the other hand, if the phase-locked-loop is off of an unstable null initially, the utilization of a sweep technique can drive the system into an unstable null, since there is no prior knowledge as to the proper direction to sweep. Consequently, while the aforesaid technique improves acquisition, there is still an uncertainty when utilizing this method.
Still another prior art technique, which operates in practice similarly to the sweep technique, is to inject noise into the system. Given that the system is initially on an unstable null, the injected noise will drive the system off of the unstable null at a predetermined time depending on when the noise is applied. Thus, theoretically, the system will be driven off of an unstable null more quickly. But unfortunately, there is always the probability that, initially, the system will be at some point between a stable null and an unstable null; therefore, there is always a probability that the injected noise will dirve the system into an unstable null rather than a stable null. This possibility is clearly depicted in FIG. 1b.
Other elaborate and sophisticated prior art systems have been devised to improve acquisition such as the "Automatic Carrier Acquisition System" of Fletcher et al., U.S. Pat. No. 3,746,998; the "Quadriphase Modem" system of Wolejsza, Jr., U.S. Pat. No. 3,594,651; and the "Automatic Signal Acquisition Means for Phase-Lock-Loop with Anti-sideband Lock Protection" of Brown et al., U.S. Pat. No. 3,768,030.
While all of the aforementioned prior art systems and techniques tend to improve the acquisition process, the uncertainties, which tend to make it difficult to predict acquisition with a degree of accuracy necessary for reliable utilization of phase-locked-loops in communication systems of the type previously mentioned, have not been eliminated.